DE 197 06 466 A1 discloses a method according to the preamble. A flow-forming roller compresses the circumferential area of a circular metal blank in such a way that in a central area of the blank material flows axially on a centrally positioned overarm support and consequently a hub is formed. The axial material flow is limited by the contour of the overarm support. The state of compressive stress, in which an axial material flow is brought about by radial pressure, is difficult to achieve. Fundamentally high forces are required. This can lead to an undesirably high material outflow and to a disadvantageously pronounced reduction in the workpiece thickness. As a result of the high forces, particularly with a high degree of forming, the risk exists of the hub being torn from the circular blank during forming.
A comparable method is disclosed by DE 44 44 526 C1. In this case a hub is formed on the tool pin of the main spindle of a flow-forming machine, the axial material flow being limited by a tail stock. The latter has a diameter larger than the tool pin diameter. As a result of the stop obtained material can only flow to the correspondingly predetermined position. The problems caused by the high compressive stresses are comparable with those of DE 197 06 466 A1.
A further hub forming possibility is disclosed by DE 196 43 820 C1. In the described method the hub is formed in such a way that an externally engaging roller initially leads to the formation of a fold, which is then compressed to the hub. This method suffers from the disadvantage that during fold formation cracking can occur if the deformability of the material is exceeded and this ultimately leads to an instability of the workpiece. A hub formed in this way has a lower strength than a solid hub with respect to subsequent stresses. When high forces are applied the fold can encourage crack propagation.
Another method for producing a hub is disclosed by DE 195 13 634 C2, where a flange-like projection of a workpiece is radially split into two "wings" and subsequently one of these wings is pressed against a chuck and the wing is quasi "turned round", so that the desired hub is obtained.
Another example for forming a hub is known from EP 824 049 A1. In this case a metal spinning roller moves the workpiece material in "wavy" manner towards the center of a workpiece, so that ultimately a hub is formed. It is disadvantageous in this method in that it only permits the production of short, thin-walled hubs. The production process is also difficult, because it is necessary to set a complex, three-dimensional state of stress.
DE 196 20 812 A1 provides a possibility of forming a hub in simple manner from a tubular workpiece. This per se adequately functioning method is consequently based o n a different starting workpiece a nd is therefore unsuitable for eliminating the aforementioned prior art disadvantages in the case of fabrication from a circular blank.
Other conventional methods for the production of hubs make use of the joining together of two or more semifinished parts. As a result of this joining together there are always manufacturing tolerances, which impair the quality of the end product. As the joining together usually takes place by welding, there is a further undesired consequence in that the workpieces are distorted as a result of th e heat influence. There is also a risk of modifications to the workpiece structure.
It is also known to produce a hub by pressing a cylindrical mandrel through a perforated preform. As a result material volume is ironed out from the inner area of the perforated preform to provide a hub. There are also restrictions to this method in connection with the hub thickness and length which can be produced.